Subject: Re: [v3 PATCH] usb: create usb_debug_root for gadget only

From: Felipe Balbi <[email-masked]>
Date: Fri, 31 May 2019 08:44:55 +0300

Hi,

Chunfeng Yun <[email-masked]> writes:

> Hi Felipe,
> On Tue, 2019-05-28 at 11:11 +0300, Felipe Balbi wrote:
>> Hi,
>> 
>> Chunfeng Yun <[email-masked]> writes:
>> > diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
>> > index 7fcb9f782931..88b3ee03a12d 100644
>> > --- a/drivers/usb/core/usb.c
>> > +++ b/drivers/usb/core/usb.c
>> > @@ -1190,7 +1190,7 @@ EXPORT_SYMBOL_GPL(usb_debug_root);
>> >  
>> >  static void usb_debugfs_init(void)
>> >  {
>> > -	usb_debug_root = debugfs_create_dir("usb", NULL);
>> > +	usb_debug_root = debugfs_create_dir(USB_DEBUG_ROOT_NAME, NULL);
>> >  	debugfs_create_file("devices", 0444, usb_debug_root, NULL,
>> >  			    &usbfs_devices_fops);
>> >  }
>> 
>> might be a better idea to move this to usb common. Then have a function
>> which can be called by both host and gadget to maybe create the
>> directory:
>> 
>> static struct dentry *usb_debug_root;
>> 
>> struct dentry *usb_debugfs_init(void)
>> {
>> 	if (!usb_debug_root)
>>         	usb_debug_root = debugfs_create_dir("usb", NULL);
>> 
>> 	return usb_debug_root;
>> }
>> 
>> 
>> Then usb core would be updated to something like:
>> 
>> static void usb_core_debugfs_init(void)
>> {
>> 	struct dentry *root = usb_debugfs_init();
>> 
>> 	debugfs_create_file("devices", 0444, root, NULL, &usbfs_devices_fops);
>> }
>> 
> I find a problem when move usb_debugfs_init() and usb_debugfs_cleanup()
> into usb common, it's easy to create "usb" directory, but difficult to
> cleanup it:
>
> common/common.c
>
> struct dentry *usb_debugfs_init(void)
> {
>     if (!usb_debug_root)
>         usb_debug_root = debugfs_create_dir("usb", NULL);
>
>     return usb_debug_root;
> }
>
> void usb_debugfs_cleanup(void)
> {
>     debugfs_remove_recursive(usb_debug_root);
>     usb_debug_root = NULL;
> }
>
> core/usb.c
>
> static void usb_core_debugfs_init(void)
> {
>     struct dentry *root = usb_debugfs_init();
>
>     debugfs_create_file("devices", 0444, root, NULL,
> &usbfs_devices_fops);
> }
>
> static int __init usb_init(void)
> {
>     ...
>     usb_core_debugfs_init();
>     ...
> }
>
> static void __exit usb_exit(void)
> {
>     ...
>     usb_debugfs_cleanup();
>     // will be error, gadget may use it.
>     ...
> }
>
> gadget/udc/core.c
>
> static int __init usb_udc_init(void)
> {
>     ...
>     usb_debugfs_init();
>     ...
> }
>
> static void __exit usb_udc_exit(void)
> {
>     ...
>     usb_debugfs_cleanup();
>     // can't cleanup in fact, usb core may use it.
> }
>
> How to handle this case? introduce a reference count? do you have any
> suggestion?

I guess a simple refcount is the way to go:

struct dentry *usb_debugfs_init(void)
{
	if (!usb_debug_root)
		usb_debug_root = debugfs_create_dir("usb", NULL);

	usb_debug_root_refcnt++;
	return usb_debug_root;
}

void usb_debugfs_cleanup(void)
{
	if (!(--usb_debug_root_refcnt)) {
		debugfs_remove_recursive(usb_debug_root);
		usb_debug_root = NULL;
	}
}

Or something along those lines

-- 
balbi

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